The future 3G LTE (Long Term Evolution) systems represent a revolutionary path of future's cellular systems based on 3GPP standards. It is presumed that LTE will have both a new radio access scheme and a new radio access network (RAN) architecture. SC-FDMA (single carrier frequency division multiple access) technique is among the most probable radio access technologies for 3G uplink (UL). The performance of SC-FDMA is not limited by the own cell interference since the system is designed to be orthogonal. On the contrary, the coverage area in orthogonal systems like OFDMA (orthogonal FDMA) or SC-FDMA is typically limited by the other cell interference.
A traditional way to reduce the amount of other cell interference is to use frequency reuse. Frequency reuse is used, for example, in GSM (global system for mobile communications)/EDGE (enhanced data rates for global evolution) systems. A problem with the frequency reuse is that it wastes the capacity of the network. Another possibility is to utilize processing gain by spectrum spreading and channel coding. This approach is widely used in 3G systems. Network capacity in 3G systems is limited by the other cell and own cell interference.
For example, 3.9G is a challenging system since the working assumption is frequency reuse 1 and at the same time significantly improved system performance in terms of average throughput (per Hz) and cell edge throughput is targeted. Interference control (IC) may be needed for fulfilling the tight performance requirements of 3.9G systems. The IC controls the radio resources by applying some restrictions to the resource management. Such restrictions in a cell will provide the possibility for improvement in SINR and cell edge data rates on the corresponding resources in neighboring cell(s).
Examples of interference control schemes for 3.9G uplink of 3GPP are described in “Uplink Interference Control Considerations, R1-050813”, RAN WG1 #42 and in “System level performance of UL SC-FDMA, R1-051411”, RAN WG1 #43.
The interference control in 3.9G uplink is of great importance. Since the exchange of physical layer information between base stations is expected to be limited, methods that rely on (beforehand) fixed principles are attractive. One such method is the user grouping principle where users in separate cells are allocated to the same frequency-time slots (reuse factor between cells is 1) according to their reported path losses. It is noted that the term ‘path loss’ has a loose meaning since the antenna gains are included in it. Hence, although physical signal paths between user terminal and transmitter antennas of adjacent cells are almost the same, the measured ‘path losses’ with respect to adjacent cells may greatly vary since base station transmit antenna gains with respect to adjacent cells are not the same. Further, the term ‘adjacent cell’ is used loosely here since it is referred to all cells for which mutual control between cells is possible. This is the case when a single base station operates a site that supports multiple sectors each having their own logical cells.
FIG. 1 shows an illustrative example of a grouping principle where frequency-time resources are divided into frequency-time resource groups in a plurality of cells 120, 122, 124 of a radio system. The plurality of cells 120, 122, and 124 may be adjacent cells but the frequency band division and radio resource allocation may also be applied to isolated cells. As illustrated in FIG. 1, the frequency band 140 division into frequency-time resource groups 130, 132, 134 may be carried out in substantially the same manner in the plurality of cells 120, 122, 124 of the radio system. User terminals within the coverage area of a plurality of adjacent cells 120, 122, 124 may be allocated to the frequency-time resource groups in a similar manner for each cell on the basis of the modulation and coding schemes used by the user terminals and/or power levels or path loss values associated with the transmitted signals of the user terminals. Thus, user terminals with substantially the same characteristics (modulation and coding and/or power levels or path loss values, for example) are usually allocated to the same or adjacent frequency-time resource group in the plurality of adjacent cells.
FIG. 2 shows a simplified illustrative example where user terminals 100-105 of cells 1-3 admit round the same path losses 200-205. The path loss quantization may vary depending on the number of user terminals. According to prior art, user terminals 100-105 are allocated to the same, for example to the first, time slot in a frame. The problem is, however, that the interference between sectors (or adjacent cells) is not taken into account. Thus, the user terminals 100 and 101, as well as the user terminals 102 and 103, and 104 and 105, are using the same frequency-time resources and interference between the sectors can become a factor that limits the achievable data rate in an area where the user terminals are close to the border of cells of the same base station. Therefore, there is a need to improve radio resource allocation processes.